Distortion

Typical for standard/normal lenses the Pentax FA 43mm exhibits only a minor degree of
barrel distortion (0.65%) - nothing to worry about under field conditions.

The chart above has a real-world size of about 120x80cm.

Vignetting

As a full format normal lens the 43mm f/1.9 enjoys a sweet spot advantage on the K10D.
Nonetheless it still produces comparatively pronounced vignetting at f/1.9. However,
the problem is negligible from f/2.8 onwards.

MTF (resolution)

The FA 43mm f/1.9 Limited produced stellar resolution figures in the MTF lab
except at wide-open aperture. At f/1.9 it suffers from rather low contrast
combined with a soft border performance whereas the center is already
pretty sharp. The situation is completely different at f/2.8 where both
resolution and contrast are extremely high. The peak quality is reached at
f/4 with an outstanding center and border quality - surely a reference quality
to be beaten on the K10D. Beyond f/4 diffraction slowly takes its toll.
The field curvature (curved focus field) is a bit pronounced with this lens.

Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Imatest Explanations

Chromatic Aberrations (CAs)

Lateral chromatic aberrations (color shadows at harsh contrast transitions) are
relatively pronounced for a "normal" lens with an average CA pixel width beyond
1px on the average at the image borders. This is not a big deal but the
(younger) DA 40mm f/2.8 Limited did a better job here.